Messages in this thread | | | Date | Wed, 26 Nov 2008 10:02:55 -0800 | From | Andrew Morton <> | Subject | Re: [PATCH 2/5] ftrace: use code patching for ftrace graph tracer |
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On Wed, 26 Nov 2008 11:46:58 -0500 (EST) Steven Rostedt <rostedt@goodmis.org> wrote:
> > > On Tue, 25 Nov 2008, Andrew Morton wrote: > > > On Wed, 26 Nov 2008 00:16:24 -0500 Steven Rostedt <rostedt@goodmis.org> wrote: > > > > > From: Steven Rostedt <rostedt@goodmis.org> > > > > > > Impact: more efficient code for ftrace graph tracer > > > > > > This patch uses the dynamic patching, when available, to patch > > > the function graph code into the kernel. > > > > > > This patch will ease the way for letting both function tracing > > > and function graph tracing run together. > > > > > > ... > > > > > > +static int ftrace_mod_jmp(unsigned long ip, > > > + int old_offset, int new_offset) > > > +{ > > > + unsigned char code[MCOUNT_INSN_SIZE]; > > > + > > > + if (probe_kernel_read(code, (void *)ip, MCOUNT_INSN_SIZE)) > > > + return -EFAULT; > > > + > > > + if (code[0] != 0xe9 || old_offset != *(int *)(&code[1])) > > > > erk. I suspect that there's a nicer way of doing this amongst our > > forest of get_unaligned_foo() interfaces. Harvey will know. > > Hmm, I may be able to make a struct out of code. > > struct { > unsigned char op; > unsigned int offset; > } code __attribute__((packed)); > > Would that look better?
nah, let's do something more generic for this.
> > > > > + return -EINVAL; > > > + > > > + *(int *)(&code[1]) = new_offset; > > > > Might be able to use put_unaligned_foo() here. > > > > The problem is that these functions use sizeof(*ptr) to work out what > > to do, so a cast is still needed. A get_unaligned32(ptr) would be > > nice. One which takes a void* and assumes CPU ordering. > > Is there a correctness concern here? This is arch specific code, so I'm > not worried about other archs.
No, the code is OK as-is.
It's just that "read a word from an [maybe-]unaligned address" is such a common operation that there should be a nice clean simple function to do it, rather than doing open-coded (and different) weird C tricks at each and\ every site.
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